foot speed, dexterity, great leg muscle endurance are some good physical attributes of good defenders. Good hip flexibility (Psoas muscle, adductor muscles) is also key as it allows for improvement and refinement of all the aforementioned attributes.

In general though, the best defenders are those that understand what their matchup's strengths are and have a great understanding of where other players (especially help defenders) are on the court. The best defenders in the league are usually the ones that watch the most tape or study the scouting reports religiously. Obviously in a pickup setting this isn't possible, but the best defenders in pickup are usually those that are able to quickly assess the strengths and weaknesses of any given player.